1. Field of the Invention
The present invention relates to methods for selectively concentrating 2,6-dimethylnaphthalene by adsorption in a mixture containing dimethylnaphthalene isomers.
2. Description of the Related Art
Polyethylene naphthalate (PEN) resins have excellent characteristics, including thermal resistance and impermeability to gases, which are not exhibited by well-known PET resins, and 2,6-dimethylnaphthalene (2,6-DMN) is a starting material for these polyethylene naphthalate resins.
Dimethylnaphthalene includes nine isomers. If 2,6-dimethylnaphthalene used for PEN contains the other isomers, the physical properties of the end product, that is, PEN, including thermal resistance and strength are degraded. Therefore, 2,6-dimethylnaphthalene must be separated out from the isomer mixture with high purity.
In particular, the physical properties of 2,6-dimethylnaphthalene are very close to those of 2,7-dimethylnaphthalene and, therefore, it is difficult to separate them. How efficiently 2,6-dimethylnaphthalene is concentrated is a technical challenge.
In order to separate out 2,6-dimethylnaphthalene, cooling crystallization methods, adsorption methods, and the like have been suggested. However, since 2,6-dimethylnaphthalene and 2,7-dimethylnaphthalene form a eutectic mixture, cooling crystallization decreases the yield of 2,6-dimethylnaphthalene and increases the cost of purification.
On the other hand, Japanese Examined Patent Application Publication No. 49-27578 discloses a method for separating out 2,6-dimethylnaphthalene by adsorption, in which a dimethylnaphthalene isomer mixture is supplied to an adsorption column packed with Y-type zeolite, serving as an adsorbent, to be concentrated by adsorption and, further, the discharged solution is separated into 2,7-dimethylnaphthalene and 2,6-dimethylnaphthalene by cooling crystallization.
Also, Japanese Unexamined Patent Application Publication No. 6-65114 suggests that a Y-type zeolite containing potassium ions be used as an adsorbent for separating out 2,6-dimethylnaphthalene. In the embodiment of the above-described Japanese Unexamined Patent Application Publication No. 6-65114, KNa-Y type zeolite pellets in which 98% of cation sites are exchanged for potassium ions are packed in a column having an inner diameter of 1.07 cm and a length of 50 cm.
However, according to these adsorption methods, the concentration of 2,6-dimethylnaphthalene in dimethylnaphthalene isomer mixtures does not reach a sufficient level. While the adsorption methods need to be improved, suitable methods for operating adsorption columns to fully bring out the performance of adsorbents have not been found yet.
Although it is known that, for example, adsorbents having a smaller grain size improve the separation performance thereof, these adsorbents increase the pressure loss of the adsorption column. Also, an adsorption column having a higher ratio of the length to the inner diameter thereof increases the linear velocity of fluid supplied to the adsorption column, thereby reducing the diffusion resistance, at the surface of the adsorbent, of dimethylnaphthalene in the adsorption column. Thus, the separation performance is enhanced; however, the pressure loss of the column increases.
Accordingly, an object of the present invention is to provide a method for fully bringing out the performance of adsorbents and selectively concentrating 2,6-dimethylnaphthalene in a dimethylnaphthalene isomer mixture.
The inventors have conducted intensive research to accomplish this object and found a factor for fully bring out the performance of adsorbents. The factor is expressed by (u1/3/xcex5)dxe2x88x925/3 (m5/3 sxe2x88x921/3 kgxe2x88x921), wherein u represents the linear velocity (m/s) of a solution of dimethylnaphthalene isomer mixture supplied to an adsorption column, xcex5 represents the packing density (kg/m3) of a Y-type zeolite adsorbent, and d represents the grain size (m) of the adsorbent.
According to an aspect of the present invention, a method for concentrating 2,6-dimethylnaphthalene in a dimethylnaphthalene isomer mixture in which o-xylene contains the 2,6-dimethylnaphthalene and 2,7-dimethylnaphthalene. The method includes the step of supplying the dimethylnaphthalene isomer mixture to an adsorption column packed with Y-type zeolite. The value of (u1/3/xcex5)dxe2x88x925/3 is set at 14 (m5/3 sxe2x88x921/3 kgxe2x88x921) or more. u represents the linear velocity (m/s) of the solution of dimethylnaphthalene isomer mixture supplied to the adsorption column, xcex5 represents the packing density (kg/m3) of the Y-type zeolite, and d represents the grain size (m) of the Y-type zeolite.
The linear velocity u (m/s) is derived from the expression u=F/S, wherein S represents the cross section (m2) of the adsorption column and F represents the flow rate (m3/s) of the fluid supplied to the adsorption column.
The packing density xcex5 (kg/m3) is derived from the expression xcex5=W/V, wherein W represents the weight (kg) of the Y-type zeolite and V represents the volume (m3) of the adsorption column.
In order to set the above-described value of the expression (u1/3/xcex5)dxe2x88x925/3 at 14 (m5/3 secxe2x88x921/3 kgxe2x88x921) or more, preferably, the linear velocity u is in the range of 1xc3x9710xe2x88x925 to 1xc3x9710xe2x88x923 m/s, the packing density xcex5 is in the range of 500 to 1000 kg/m3, and the grain size d is in the range of 1xc3x9710xe2x88x924 to 5xc3x9710xe2x88x923 m.
Preferably, at least part of ion-exchanging sites of the Y-type zeolite are exchanged for potassium ions.